In industry the use of metal products manufacture by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced and the quality requirements are continuously raised at the same time as it is desired to reduce costs. The powder metallurgy (PM) technology enables a cost effective production of components, especially when producing complex components in long series, as net shape or near net shape components can be manufactured without the need of costly machining. A drawback however with the PM technology is that the sintered parts will exhibit a certain degree of porosity which may negatively influence the mechanical properties of the part. The development within the PM industry has therefore been directed to overcome the negative influence of the porosity basically along two different development directions.
One direction is to reduce the amount of pores by compacting the powder to higher green density (GD) facilitating sintering to a high sintered density (SD) and/or performing the sintering under such conditions that the green body will shrink to high SD. The negative influence of the porosity can also be eliminated by removing the pores at the surface region of the component, where the porosity is most harmful with regards to mechanical properties, through different kinds of surface densification operations.
Another development route is focused on the alloying elements added to the iron-based powder. Alloying elements may be added as admixed powders, fully pre-alloyed to the base iron powder or diffused to the surface of the base iron powder. Commonly used alloying elements are besides carbon, which is normally admixed in order to avoid a detrimental increase of the hardness and decrease of the compressibility of the iron-based powder, copper, nickel, molybdenum and chromium. The cost of alloying elements however, especially nickel, copper and molybdenum, makes additions of these elements less attractive. Copper will also be accumulated during recycling of scrap why such recycled material is not suitable to be used in many steel qualities where no or a minimum of copper is required.
Iron-based powders having low amounts of alloying elements without nickel and copper are previously known from e.g. the U.S. Pat. Nos. 4,266,974, 5,605,559, 5,666,634, and 6,348,080.
The purpose of the invention according to U.S. Pat. No. 4,266,974 is to provide a powder satisfying the demand of high compressibility and to provide a sintered body having good hardenability and good heat treatment properties. According to this prior art document, the most important step in the production of the steel alloy powder produced according to this prior art method is the reduction annealing step.
The U.S. Pat. Nos. 5,605,559 and 5,666,634 both concern steel powders including Cr, Mo and Mn. The alloy steel powder according to the U.S. Pat. No. 5,605,559 comprises, by weight, about 0.5-2% Cr, not greater than about 0.08% of Mn, about 0.1-0.6% of Mo, about 0.05-0.5% of V, not greater than about 0.015% of S, not greater than about 0.2% of O, and the balance being Fe and incidental impurities. The U.S. Pat. No. 5,666,634 discloses that the effective amounts should be between 0.5-3% of chromium, 0.1-2% by weight of molybdenum and at most 0.08% by weight of manganese.
A serious drawback when using the invention disclosed in the U.S. Pat. Nos. 5,605,559 and 5,666,634 is that cheap scrap can not be used as this scrap normally includes more than 0.08% of manganese. In this context the U.S. Pat. No. 5,605,559 teaches that “when Mn content exceeds about 0.08% wt, oxide is produced on the surface of alloy steel powders such that the compressibility is lowered and hardenability increased beyond the required level . . . . Mn content is preferably not greater than about 0.06% wt (col 3, 47-53).
The U.S. Pat. No. 5,666,634 refers to a Japanese Laid-Open No. 4-165 002 which concerns an alloy steel powder including in addition to Cr also Mn, Nb and V. This alloy powder may also include Mo in amount above 0.5% by weight. According to the investigations referred to in the U.S. Pat. No. 5,666,634, it was found that Cr-based alloy steel powder is disadvantageous due to the existence of the carbides and nitrides which act as sites of fracture in the sintered body.
The U.S. Pat. No. 3,725,142 discloses an atomized steel powder having improved hardenability. However, improved hardenability is in this case achieved by intentional additions of boron. “According to the invention boron is added to the melt in amount of 0.005-0.100 percent by weight and preferably in the range of 0.0075-0.0500 percent by weight” (col 2, 59-62). Alloying with boron at such low additions not only creates problems regarding reproducibility, but also requires adaptation of the standard water atomizing process in order to ensure success (as described in Col3, 27-65), thus increasing production cost.
The possibility of using powders from scrap is disclosed in the U.S. Pat. No. 6,348,080 which discloses a water-atomised, annealed iron-based powder comprising, by weight % Cr 2.5-3.5, Mo 0.3-0.7, Mn 0.09-0.3, O<0.2, C<0.01 the balance being iron and, an amount of not more than 1%, inevitable impurities. This patent also discloses a method of preparing such powder. Additionally, the U.S. Pat. No. 6,261,514 discloses the possibility of obtaining sintered products having high tensile strength and high impact strength if powders having a composition as disclosed in U.S. Pat. No. 6,348,080 is warm compacted and sintered at a temperature above 1220° C.
The international patent application WO 03-106079 describes a low alloyed steel powder having an amount of chromium between 1.3 to 1.7% by weight, molybdenum between 0.15-0.3%, manganese between 0.09-0.3%, not more than 0.01% of carbon and not more than 0.256% by weight of oxygen. It is further taught that nickel and/or copper may be admixed to the powder or adhered to the surface of the powder by using a bonding agent or being diffusion bonded to the surface.
It is stated in the WO application 03-106079 that the maximum allowable partial pressure of oxygen is 5×10−18 atm in the sintering atmosphere when sintering green components produced from compacted powders as described in U.S. Pat. No. 6,348,080, whereas the corresponding value for allowable partial pressure of oxygen for the sintering atmosphere is 3×10−17 atm when sintering components made of powders according to WO 03-106079. Nothing else is taught about the sintering atmosphere but due to the very low partial pressures of oxygen, the in PM production normally used Endogas atmosphere is not suitable due to its high partially pressure of oxygen. The choice of atmospheres during sintering is therefore limited to more expensive hydrogen containing atmospheres such as 100% of hydrogen or hydrogen mixed with nitrogen for example 90% hydrogen/10% of nitrogen.
Hence, there is a need of an iron-based alloyed steel powder having lower amounts of costly alloying elements, suitable to be compacted into green components which may be sintered in atmospheres having relatively high partial pressures of oxygen such as the Endogas normally used in the PM industry.
It has now surprisingly been found that a Cr/Mo/Mn/Ni containing iron-based alloyed steel powder can suitably be used for producing compacted and sintered parts having a sufficiently high mechanical strength after heat treatment in an Endogas atmosphere comparable to parts produced from powders according to the MPIF standard FN 0205 or FLN2-4405-HT. The new powder may also be sintered in an Endogas atmosphere having relatively high partial pressure of oxygen. According to the present invention other gases than Endogas can be used if the gas atmosphere has a partial oxygen pressure similar to the partial oxygen pressure in Endogas and if the gas can be produced at a relatively low price. Endothermic gas (Endogas) is a blend of carbon monoxide, hydrogen, and nitrogen with smaller amounts of carbon dioxide water vapour, and methane produced by reacting a hydrocarbon gas such as natural gas (primarily methane), propane or butane with air. For Endogas produced from pure methane, the air-to-methane ratio is about 2.5; for Endogas produced from pure propane, the air-to-propane ratio is about 7.5. These ratios will change depending on the composition of the hydrocarbon feed gases and the water vapour content of the ambient air. Endogas is produced in a special generator by incomplete combustion of a mixture of fuel gas and air, using a catalyst. It is possible to produce an Endogas atmosphere having a partial pressure of oxygen of about 10−15 to 10−16 which partial pressure of oxygen is sufficient to allow sintering of the new material.